The present invention relates to a circuit arrangement and a device utilizing said circuit arrangement, and more particularly a differential circuit arrangement. The circuit arrangement described herein is, in one or more embodiments, utilized with small photo currents and for detection of small photo currents.
The discussion of the background provided herein is included to provide context. This is not to be taken as an admission that any of the information was published, or was prior art.
A Wheatstone bridge circuit is well suited for measuring small changes in resistance. The Wheatstone bridge circuit arrangement, as shown in FIG. 1, includes three known resistances (two are constant, R1 and R2 and one is variable, R3), an unknown variable resistor (Rx) and a voltage source. This circuit configuration is designed to provide output from a single current or voltage source. The unknown variable resistance, Rx, is found by an equation, in which Rx=(R2·R3)/R1 (R1 and R2 together providing one half of the bridge, and R3 and Rx together providing another half of the bridge). After balancing the two arms of the bridge (in which Vout=0 between two arms of the bridge), a small change in resistance of Rx causes a voltage output. In this way, small resistance changes in the unknown variable resistor can be measured. However, when applied with certain sensing devices, such as photodiodes, the photodiodes are arranged so that current generated by the photodiodes are in the same direction on each arm, meaning the diodes are arranged so as to face the same direction. Furthermore, because photodiodes produce only very small current flow even when fully lit, when photodiodes are in such an arrangement, the circuit will generally require additional input (active elements). This is often accomplished by at least adding an operational amplifier and/or a power source. The operational amplifier is typically arranged to provide feedback and/or to obtain an output voltage. For example, in typical configurations, at least one operational amplifier and often more than one operational amplifier are included, these configurations often having one operational amplifier associated with each photodiode. The operational amplifier will generally include two input terminals to operate the photodiode without bias. A capacitor is often included to prevent oscillation or gain peaking and to set output bandwidth. These add-on or active elements add to complexity of the circuit.
It is also difficult to obtain linear output from a photodiode in a Wheatstone bridge configuration. While, the addition of the external operational amplifiers to the circuit assist in obtaining linear output, such a circuit still requires even further external components to provide a linearizing circuit. For example, a bipolar power supply can be included. Alternatively, a unipolar power supply is added. Again, the additional external (active) sources add to complexity of the circuit.
The typical photodiode circuit arrangements described above not only have added complexity, the additional or external (active) elements add bulk. Further, it is found that with any of the above-described complex circuit arrangement, self-heating and/or overheating will likely occur, which can affect measurement accuracy.
In view of the above issues and problems, there remains a need to overcome said issues and problems as well as others associated with such circuit arrangements. There remains a need for improved circuit arrangements, ones for detecting and measuring small photo currents and for use with photocurrent devices.